"Dear Friends," she began. "Tonight ... you have watched our last and final weekly episode. My gratitude and appreciation to all of you ... who along with me have solved 264 murder mysteries over 12 great years." She signed it, "With love."

Her affectionate farewell signals a double loss. It marks not only the end of a popular whodunit, but also the disappearance from TV of a mature actress beloved for her portrayal of a smart, classy mystery writer.

Ms. Lansbury's departure comes just weeks after a study reveals the minimal presence of women like her in television programming. Called "Virtually Invisible: The Image of Midlife and Older Women on Prime Time TV," the report offers an overview of women in entertainment roles, from 1973 through 1994. Conducted by the University of Pennsylvania's Annenberg School of Communication, it was sponsored by the American Association of Retired Persons.

The majority of characters portrayed on prime-time television continue to be young and male, according to the report. Despite a recent increase in the presence of young women, roles for midlife and older women remain limited and stereotypical. While older male characters tend to be defined by their occupation, older women are defined by their marital status and their relationship to family.

Women 65 and over receive the least attention. Even when they do appear, they're often portrayed as ineffective or weak. Only a quarter of major female characters in this age group are cast in positive roles, as "heroes," compared with more than half of their male counterparts. "The presentation of highly effective older women in network television entertainment is indeed a rare phenomenon," the report states.

When researchers analyzed 50 prime-time scripts treating older characters in some depth, they found more than half of older women being ridiculed, while only 20 percent of older men endure such indignities. Many shows treat aging as a target for one-liners about isolation and infirmity. Even "The Golden Girls" "follows the television formula for depicting older women and in some cases exceeds it in bias and meanspiritedness." Its four characters "insult, demean, and ridicule each other with impunity, rarely exchanging words of affection or respect."

Women's invisibility in mass media is hardly limited to midlife and older women in TV entertainment. Last month, an annual study by Women, Men, and Media reported that women are the subject of only 15 percent of front-page newspaper articles, down from 19 percent last year.

The latest Harper's Index also offers this sobering statistic: "Number of last year's newsweekly magazine covers featuring women who are not princesses, murderers, or models: 0."

It's easy to argue that prime-time TV is "just entertainment." But given television's huge audiences, its powerful role in shaping attitudes and stereotypes cannot be minimized. How can viewers, especially young viewers, understand the capabilities and strengths of women in midlife and beyond if they are constantly "entertained" with negative images?

The same holds true for print journalism. What does it imply about the status of everyday women of all ages in the real world if women remain largely invisible or are portrayed only as problems - as welfare mothers on Page 1 of newspapers, for example - or as "princesses, murderers, or models" on magazine covers?

The authors of the TV study call their findings "troubling given the significant role television plays in social learning. Greater awareness and dialogue are necessary if the potential of television to enlighten as well as entertain is to be fulfilled."